Specimens of baboon placentae which span the time from approximately the second week of gestation through term are to be studied at the ultrastructural level for the developmental sequence in the structure of (1) the placental barrier and (2), the changes in the endometrial stroma (decidua). Special emphasis is being placed on the structure and changes in the structure which take place in the spiral arteries which supply maternal blood to the intervillous space. This material is being compared with tissues from human placentae of similar developmental stages in an effort to identify any morphological dissimilarities which might help explain why non-human primates such as the baboon do not normally develop the hypertensive diseases of pregnancy (pre-eclampsia and eclampsia) which are common to human pregnancy. BIBLIOGRAPHIC REFERENCE: Houston, M.L. and A.G. Hendrickx Placental Structure of the Sooty Mangabey (Cercocebus Atys) Anat. Rec. 181: 379 (1975).